1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance apparatus for identifying either spectra or images of an examination subject. The apparatus is of the type using a whole-body antenna to irradiate the examination subject with a sequence of radio-frequency pulses, and a surface coil for acquiring the nuclear magnetic resonance signals emitted by the examination subject. The whole-body antenna and the surface coil are decoupled from each other.
2. Description of the Prior Art
Surface coils are often utilized for the examination of individual body regions of an examination subject using nuclear magnetic resonance principles. Such surface coils improve the spatial resolution in the sensitivity region of the surface coil, due to a rise in the signal-to-noise ratio. Transmission is preferably undertaken using a whole-body antenna covering the entire examination volume in order to generate an optimally homogenous signal in the region of the surface coil with respect to the radio-frequency excitation. The signals emitted by the resonated nuclei are then received with the surface coil, at which time the homogeneity plays only a subordinate role.
Because the whole-body antenna and the surface coil are tuned to the same resonant frequency, these elements must be mutually decoupled to prevent one element from influencing the other. When the whole-body antenna generates a linearly polarized field, the field of the surface coil can be aligned perpendicularly to the field of the whole-body antenna. The physical position of the surface coil, however, is fixed relative to the whole-body antenna and the surface coil cannot be freely oriented, for example, for adaptation to a particular anotomical environment. Given a circularly polarized whole-body antenna, a decoupling of the surface coil and the whole-body antenna in this manner is not possible.
An article by Boskamp in Radiology, Vol. 157, No. 2, pages 449-452 discloses detuning of the surface coil by the use of tuning diodes (Varicaps) while the whole-body antenna is in operation. During the operation of the surface coil, the whole-body antenna is shorted with the assistance of a diode bridge consisting of P-I-N diodes.
In this known detuning method, however, there is a risk that the measures provided for decoupling will influence the whole-body antenna during the transmission or reception mode, and there is the further risk that the decoupling measures will deteriorate the transient circuit quality.
Moreover, it is not sufficient to achieve decoupling of the surface coil simply by tuning toward higher frequencies. A higher voltage, which can destroy the tuning diodes, arises at the high resonant impedance due to the radio-frequency coupling-in of the whole-body antenna.
A circuit arrangement for decoupling the individual coils of a multiple coil measuring head or probe in a nuclear magnetic resonance apparatus is disclosed in German OS No. 34 27 666. In this circuit, a transmission coil is connected to a transmitter via a .lambda./2 line and via two parallel branches of diodes, each branch consisting of two diodes in series and the branches being connected with the polarities of the diodes therein in opposite directions. The reception coil is connected via a .lambda./4 line to a receiver which has an input with an anti-parallel circuit of two diodes connected in parallel therewith.
During transmission, the reception coil generates a high output voltage which exceeds the thresholds of the diode connected in parallel with the receiver input, so that these diodes become conductive and the voltage across the circuit is limited to the threshold voltage.
During reception, the diodes connected in parallel with the receiver input are non-transmissive because of the significantly weaker voltages. The signals registered during the reception mode of the transmission coil similarly do not reach the thresholds of the diodes preceding the transmitter.
Thus, in this circuit as well, a complete decoupling is not guaranteed on the basis of the diode threshold voltages.